Apparatus and method for measuring chromatic dispersion by variable wavelength

ABSTRACT

An apparatus and method for characterizing or measuring chromatic dispersion in an optical fiber segment. The fiber is accessed at one end with a signal generator injecting modulating pulsed signals which are reflected at the other end of the fiber, the apparatus determining the phase difference between the modulation signals and the reflected modulation signals and calculating the chromatic dispersion therefrom.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a national stage of PCT/IT02/00223 filed 9 Apr. 2002.

FIELD OF THE INVENTION

The present invention relates to an apparatus for measuring ChromaticDispersion (CD) in optical fibers, and to the related method.

BACKGROUND OF THE INVENTION

Chromatic dispersion is a particularly important characteristic ofoptical fibers due to the distortion effects it causes on the opticalsignals that propagate in fibers for telecommunications.

Such a phenomenon is, as is well known, linked to the different groupvelocities with which various spectral components of optical signalspropagate in a fiber, for instance between an optical transmitter andreceiver or a first and a second end of the fiber.

The spectral components of an optical signal, due to the different groupvelocities, reach the receiver at different times and determine adistortion of the received signal both in analogue transmission systems,and in digital transmission systems.

The chromatic dispersion of optical fibers, as is well known, isdetermined mainly by two factors, the dispersion characteristics of thematerial whereof the fibers are made (dependence of the refractive indexof the material on frequency), and the very nature of the propagation ofan optical signal in a waveguide (the so-called “waveguide dispersion”).

The chromatic dispersion of optical fibers can be suitably controlled bya careful design of the profile of the refractive index of the fibers.In fact, today, optical fibers are manufactured with chromaticdispersion characteristics specifically studied to meet the requirementsof the most sophisticated optical transmission systems.

As these requirements become ever more stringent, in particular fortransmission systems with bit rates of 10 Gbit/s or higher, the needalso increases to “characterize” the CD of the optical fibers withaccuracy, not only in the factory during the product qualificationstage, but also in field, for new or less recently installed systems.

Such “characterization” is indispensable in order to design and installnew-generation transmission systems (with bit rates of 10 Gbit/s orhigher) on recent fibers or to verify, in case of less recent systems,the possibility of supporting a higher transmission capacity.

Today, the chromatic dispersion of optical fibers is measured by meansof at least three different, consolidated techniques, as described, forinstance, in the ITU-T Recommendation G.650. Amongst such techniques,one of the most commonly used is is the so-called Phase Shift (or PS)technique. It consists of measuring the phase shift introduced by theoptical fiber segment being measured on a sinusoidal signal thatmodulates an optical carrier which is made to propagate in the fiberitself. The measurement is repeated at different wavelengths of theaforesaid optical carrier and for each of them the group delay of themodulating sinusoidal signal, which is proportional to the aforesaidphase shift, is calculated.

An apparatus or instrument for measuring chromatic dispersion is, forinstance, described in U.S. Pat. No. 6,313,934.

This document describes, inter ails, a methodology for measuringchromatic dispersion wherein the phase shift of the sinusoidal signal,that modulates the optical carrier, introduced by the fiber segment ismeasured by synchronizing, by means of absolute timing systems obtainedfrom a GPS (Global Positioning System), appropriate devices forgenerating and measuring the sinusoidal signal, positioned at the endsof the fiber itself.

The prior art method entails the need to simultaneously access the twoends of the fiber to be characterized, to apply appropriateinstrumentation to the two ends of the fiber and to use an absolutereference system with which to synchronize the instrumentation at thetwo ends of the fiber.

OBJECT OF THE INVENTION

It is the object of the present invention to provide a measuringapparatus and related method that does not necessarily require accessingthe two ends of the fiber in order to perform chromatic dispersionmeasurements, nor require an absolute reference system to synchronizethe instrumentation.

SUMMARY OF THE INVENTION

This object is achieved by an apparatus for measuring the chromaticdispersion of optical fibers which comprises an optical source able togenerate optical signals at variable wavelength, a signal generator ableto generate modulation signals, a modulator able to generate modulatedoptical signals on the basis of said optical signals and of saidmodulation signals and coupler able to send said modulated opticalsignals to a first end of said fiber. According to the invention, thesignal generator comprises means able to generate impulsive electricalsignals having variable amplitude, and duration and periodicitydetermined according to the characteristics of the fiber. The fibercomprises in correspondence with a second end a reflecting element ableto reflect the modulated optical signals and to generate reflectedoptical signals having a reflected modulation modulated component.Comparison means associated with the first end of the fiber and is ableto measure the phase difference between the modulation signals and thereelected modulation component.

The coupler comprises means able to receive said reflected opticalsignals. The comparison means comprises an optical receiver connected tothe coupler and able to convert the reflected optical signals intoelectrical signals representative of the reflected modulation component.A phase comparator of the comparison means is connected to the signalgenerator and to the optical receiver and is able to generate anelectrical signal representative of the phase difference.

Control means is associated respectively with the optical source andwith the generator and is able selectively to control the wavelength ofsaid optical signals and the characteristics of said modulation signals.

The control means can comprise computing means to calculate thechromatic dispersion (CD) of the optical fiber on the basis of the phasedifference measured as the wavelength of said optical signal varies.

The method of the invention comprises the steps of:

generating optical signals at variable wavelength;

generating modulation signals shaped by impulse electrical signalshaving predetermined phase, variable amplitude, and having duration andperiodicity determined according to the characteristics of the fiber;

modulating the optical signals with the modulation signals;

sending the optical signals modulated with the modulation signals to afirst end of the fiber;

reflecting at a second end of the fiber the modulated optical signals insuch a way as to obtain reflected optical signals having a reflectedmodulation modulated component; and

measuring in correspondence with the first end the phase differencebetween the modulation signal and the reflected modulation modulatedcomponent.

The method can comprise the additional step of:

calculating the chromatic dispersion of the optical fiber on the basisof the phase difference measured as the wavelength of the opticalsignals varies.

In particular, therefore, the object is achieved by the apparatus thatallows to characterize or measure the chromatic dispersion or CD of afiber segment accessing only one end of the fiber itself.

According to another characteristic of the present invention, theapparatus does not require absolute external reference signal, but haswithin it all that is necessary to determine the chromatic dispersion ofthe fiber being measured.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other characteristics of the present invention shall becomereadily apparent from the following description of a preferredembodiment, given with the aid of the accompanying drawing, in which:

FIG. 1 shows a block diagram of the apparatus for measuring chromaticdispersion according to the invention; and

FIG. 2 shows an example of impulsive signal used in the apparatus ofFIG. 1 to measure chromatic dispersion.

SPECIFIC DESCRIPTION

With reference to FIG. 1, the apparatus or instrument 10, according tothe invention, comprises a tuneable source of optical signals (opticalsource) 11, a device for generating modulating pulses (signal generator)12 able to modulate the optical signals generated by the optical source11, by means of a modulator or multiplier device 19, and a couplingdevice 15 able to convey the modulated optical signal on an opticalfiber 50 to be subjected to characterization or analysis.

The apparatus 10 further comprises a phase comparator 14 connected atthe output of an optical receiver 16 and a control processor (processor)18 connected by means of respective control wires (shown in the figurewith dashed lines) to the optical source 11, to the signal generator 12,to the phase comparator 14 and to the optical receiver 16.

The apparatus 10 is connected, by means of the coupling device 15, to afirst termination of the fiber (fiber segment) 50 in correspondence withthe end adjacent to the apparatus.

According to the present embodiment, the fiber segment 50 to becharacterized or measured has in correspondence with the second end, ingeneral far from the apparatus 10, a reflecting termination 51, of aknown kind, constituted, for example, by a reflecting optical component,specifically connected to the end of is the optical fiber to perform thecharacterization, or, preferably, by a not-angled terminal connector.

The optical source 11 of the apparatus 10, known in itself, is able togenerate optical signals at variable wavelength, based on commandstransmitted by the processor 18 through the respective control wire andto transmit them to the modulator device (modulator) 19 to be modulatedwith the modulating pulses.

The signal generator 12. comprises a generator of sinusoidal signals(sinusoidal generator) 23, a generator of impulse signals (impulsegenerator) 24 and an associated multiplier device 29, known inthemselves, and is able to generate the product of the aforesaidsinusoidal signal and of the impulse signal and to send them to themodulator 19 as modulation signals of the kind shown by way of examplein FIG. 2.

In particular, the modulation signals (modulating) are shaped by pulsesof determined duration and periodicity in which the pulses themselveshave their amplitude variable in sinusoidal fashion about a referencelevel.

Both the sinusoidal generator 23 and the impulse generator 24 arecontrolled by means of respective control wires by the processor 18 toenable varying both the characteristics of the sinusoidal signals and ofthe impulses.

Moreover, both the sinusoidal generator 23 and the impulse generator 24are connected, by means of respective connections, to the phasecomparator 14 and are able to transmit the respective signals to thecomparator 14 itself.

The coupling device 15, known in itself, for instance constituted by adirectional coupler or by an optical circulator, is able to collect themodulated optical signal coming from the multiplier 19 and to send it tothe optical fiber 50 through the first termination.

The coupling device 15 is also able to collect the signals reflectedback by the reflecting termination 51 and to send them, through acorresponding optical connection, to the optical receiver 16.

The optical receiver 16, of a known kind, has its input connected to thecoupling device 15 and the output connected to the phase comparator 14and it is able to detect the signals reflected back and to convert theminto electrical signals able to be measured by the phase comparator 14.

The phase comparator 14, of a known kind, is able to measure the phasedifference between the signal coming from the sinusoidal generator 23and the one received by the optical receiver 16 through the fibersegment 50.

In particular, the phase comparator 14 is able to measure the phasedifference between the continuous sinusoidal signal, generated by thesinusoidal generator 23, and the pulsed sinusoidal signal reflectedback, received by the optical receiver 16 and to transmit suchinformation, through a corresponding connection, to the processor 18.

The processor 18, of a known kind, is able, on the basis of programsdeveloped during the design of the apparatus 10, to control thedifferent devices of the instrument 10, and in particular the opticalsource 11, the impulse generator 24, the sinusoidal generator 23, thephase comparator 14 and the optical receiver 16 and to calculate anddisplay the results of the measurements of the phase difference andgroup delay as a function of wavelength, based on the information fromthe phase comparator 14.

In particular, the processor 18 is able to adjust the duration andrepetition frequency of the pulses generated by the pulse generator 24on the basis of the characteristics of the fiber segment 50 beingmeasured, such as length, presence of any intermediate connectorsbetween the first termination and the reflecting termination 51, etc.

The duration of the impulses (FIG. 2), for instance, is determined bythe processor 18 (FIG. 1, FIG. 2) in such a way that it is no greaterthan twice the time of propagation of the pulses themselves in the fibersegment 50 being measured. In the same way, the periodicity orrepetition period of the impulses, for instance, is determined by theprocessor 18 in such a way that it is no less than 4 times thepropagation time between the two ends in the fiber segment 50 beingmeasured.

The conditions indicated above for the duration of the impulses and therepetition period of the impulses themselves are, as the person skilledin the art will readily comprehend, such as to allow the phasecomparator 14 to selectively measure the phase difference between thesinusoidal signal generated by the sinusoidal generator 23 and thesinusoidal signal reflected by the reflecting termination 51.

The processor 18 is also able to inhibit the operation of the phasecomparator 14, by means of the respective control connection, during thereflections from all optical connectors (including the connector at thefirst termination) present in the fiber segment 50 being measured. Thisinhibition, in particular, is effected in synchrony with the impulsesgenerated by the impulse generator 24 thanks to the connection betweenthe pulse generator 24 itself and the phase comparator 14.

The processor 18 is also able to control the optical receiver 16, bymeans of the respective control connection, adjusting some operatingparameters such as gain, bandwidth, etc., according to thecharacteristics of the signals received by the optical receiver 16itself.

The operation of the apparatus 10 according to the invention is asfollows.

The modulated optical signal, generated by means of the combination ofthe signals coming from the optical source 11 and of the signalgenerator 12 (FIG. 2) is sent to the first end of the fiber 50 throughthe coupling device 15 and propagates to the opposite end of the fiberwhere it is reflected by the reflecting termination 51 and returns tothe coupling device 15.

The coupling device 15 sends the optical signal reflected by thereflecting termination 51 to the optical receiver 16, where it isconverted into an electrical signal and transmitted to the phasecomparator 14 for the measurement of the phase difference between thesinusoidal signal generated locally by the sinusoidal generator 23 andthe one reflected by the reflecting termination 51.

The phase shift between the two sinusoidal signals is proportional, asis well known, to the group delay of the fiber 50 at the workingwavelength of the optical source 11 and, therefore, by repeating theoperations described above with a determined number of optical signalsof various wavelengths it is possible to calculate, in a known manner,by means of the processor 18, the chromatic dispersion CD of the fibersegment 50.

Thanks to a first characteristic of the present invention, the apparatusor instrument 10 allows certifying or measurement of the chromaticdispersion CD of a fiber segment 50 accessing a single end of the fiber50 itself.

This considerably simplifies and abbreviates the measuring procedure.

Moreover, the measurements can be performed, at least in the case inwhich the fiber 50 is terminated at the second end with a not-angledconnector, by a single operator instead of two as is the case with knowninstruments.

According to an additional characteristic of the present invention, thereference signal for measuring the phase difference is available insidethe instrument and need not be obtained from a GPS receiver as in theknown case taken as a reference.

The invention was described taking as reference impulse modulationsignals, variable in amplitude in sinusoidal fashion, but, as a personversed in the art will readily comprehend, the amplitude variations ofthe impulse signals can have any shape, for instance triangular orsquare, as long as they are such as to allow a phase shift measurementbetween the generated signal and the reflected signal.

Obvious modifications or variations are possible to the abovedescription, in the dimensions, shapes, materials, components, circuitelements, connections and contacts, as well as in the details of thecircuitry and of the illustrated construction and of the method ofoperation, without thereby departing from the spirit of the invention asspecified in the claims that follow.

1. An apparatus for measuring the chromatic dispersion of an opticalfiber having first and second opposite ends, the apparatus comprising:an optical source able to generate optical signals at a variablewavelength; a signal generator able to generate modulation signals; amodulator able to generate modulated signals on the basis of the opticalsignals and of the modulation signals; a coupling device able to sendthe modulated signals to the first end of the fiber; an impulsegenerator in the signal generator and able to generate impulsiveelectrical signals having variable amplitude and having duration andperiodicity determined according to characteristics of the fiber suchthat the modulated signals are shaped by pulses having variableamplitude; a reflecting element at the second end of the fiber able toreflect the modulated signals and to generate reflected optical signalshaving a reflected modulated component; and a comparator associated withthe first end of the fiber and able to measure a phase differencebetween the modulation signals and the reflected modulated component. 2.The apparatus defined in claim 1 wherein the coupling device is furtherable to receive the reflected optical signals and the comparatorcomprises an optical receiver connected to the coupling device and ableto convert the reflected optical signals into electrical signalsrepresentative of the reflected modulated component; and a phasecomparator connected to the signal generator and to the optical receiverand able to generate an electrical signal representative of the phasedifference.
 3. The apparatus defined in claim 2 further comprising aprocessor associated with the optical source and with the signalgenerator and able selectively to control the wavelength of the opticalsignals and the characteristics of the modulation signals.
 4. Theapparatus defined in claim 3 wherein the processor is further able tocalculate the chromatic dispersion of the optical fiber on the basis ofthe phase difference measured as the wavelength of the optical signalvaries.
 5. The apparatus defined in claim 1 wherein the amplitude of thepulses is variable in sinusoidal fashion.
 6. The apparatus defined inclaim 1 wherein a duration of the pulses is no greater than twice a timeof propagation of the pulses in the fiber.
 7. The apparatus defined inclaim 1 wherein a periodicity of the pulses is no less than four times atime of propagation of the pulses in the fiber.
 8. A method formeasuring the chromatic dispersion of an optical fiber having first andsecond opposite ends, the method comprising the steps of: generatingoptical signals at variable wavelength; generating modulation signalsshaped by impulse electrical signals having predetermined phase,variable amplitude, and duration and periodicity determined according tocharacteristics of the fiber; modulating the optical signals with themodulation signals such that the optical signals modulated with themodulation signals are shaped by pulses having variable amplitude;sending the modulated signals to the first end of the fiber; reflectingat the second end of the fiber the modulated signals in such a way as toobtain reflected optical signals having a reflected modulated component;measuring at the first end a phase difference between the modulationsignal and the reflected modulated component.
 9. The method as claimedin claim 8 further comprisign the step of calculating a chromaticdispersion of the optical fiber on the basis of a phase differencemeasured as a wavelength of the optical signals varies.
 10. The methoddefined in claim 8 wherein an amplitude of the pulses is variable insinusoidal fashion.
 11. The method defined in claim 8 wherein a durationof the pulses is no greater than twice a time of propagation of thepulses in the fiber.
 12. The method defined in claim 8 wherein aperiodicity of the pulses is no less than four times a time ofpropagation of the pulses in the fiber.